Process for regenerating electrolytic solutions obtained in the electrolytic production of manganese dioxide

ABSTRACT

AQUEOUS ELECTROLYTIC SOLUTIONS CONTAINING MANGANESE SULFATE, CALCIUM SULFATE AND SULFURIC ACID, OBTAINED IN THE ELECTROLYTIC PRODUCTION OF MANGANESE (IV)-OXIDE AT ELEVATED TEMPERATURES IN ELECTROLYTIC CELLS ARE REGENERATED. THE ELECTROLYTIC SOLUTION IS WITHDRAWN FROM THE ELECTROLYTIC CELL, COOLED DOWN TO TEMPERATURES AT LEAST 5 CENTIGRADE DEGREES LOWER THAN THE ELECTROLYSIS TEMPERATURE, PRECIPITATED MATTER IS ISOLATED THEREFROM AFTER A PERIOD OF AT LEAST 15 MINUTES, AND THE SOLUTION IS RECYCLED TO THE ELECTROLYTIC CELL.

v Dec. 28, 1971 E. PREISLER E A 3,530,362

PROCESS FOR REGENERATING ELECTROLYTIC SOLUTIONS OBTAINED IN THE ELECTROLYTIC PRODUCTION OF MANGANESE DIOXIDE Filed Feb. 16, 1970 5 Sheets-Sheet 1 g (la/L 1.65 m MnSO 0-73m Muse 0 m Mwso 0.27m MnSO QIIIIIIIII I 20 4O 6O 80 100 Temperature E Temperaiure coef icierzi 0f solubz'ldygf calcium sulzfaie dilaydr z'n/ manganese suyaie so uh'oms of various concenlr'aiiorw ("FMOl/L) 1971 I 'E.'PREISLER ETAL 3,630,862 PROCESS FOR REGENERATING ELECTROLYTIC SOLUTIONS OBTAINED IN THE ELECTROLYTJC PRODUCTION OF MANGANESE DIOXIDE Filed Feb. 16, 1970 5 Sheets-Sheet 2 Ca/L 9 A 6 m N gsg 04% 0.70 m Na so 0.05 mM s0 0.7- 0.20 rwMg 30 mm- J O 6 I I I I 2O 6O 6O 80 100 Temperature I [0 Temperalw'e coeffz'a'erzl q/ solubility of calcium Sal hie dz'lzydraie z'zzsodzlzm, and magnesium s faie soluiions of various conceniralimzs [mmwZ/l) 1971 E. PREYISLER TAL 3,630,862

PROCESS FOR REGENERATING ELECTROLYTIC SOLUTIONS OBTAINED IN THE ELECTROLYTIb PRODUCTION OF MANGANESE DIOXIDE -F1led Feb. 16, 1970 5 Sheets-Sheet 3 20 a0 e0 50 100 Temperature Po] Temperczlure coefficz'eni of solubddy qf calcium Sufale dz'lq draie in solulions lam/z z /qzm'ow suffun'c acid Gardenia [m mall/L DeC. 28, I E R ETAL PROCESS FOR REGBNERATING ELECTROLYTIC SOLUTIONS OBTAINED IN THE ELECTHOLYTIG PRODUCTION OF MANGANESE DIOXIDE Flled Feb. 16, 1970 5 Sheets-Sheet 4 I I I I I I I I 0.2 0.11 0.6 0.8 1.0 1.2

Molartty NeuiraLSulfate Influence of he concenlrailbm of neuiral sulfates (Na S0 ,Mg\50 ,Mn50 upon the solubilii yzy calcium sulfaie di/Lydrale im solulioms having Varz'aus sulfuric acid coniemfs a! 70C. (m=m0L/L) Dec. 28, 1971 EQPREISLER ET AL PROCESS FOR REGENERATING ELECTROLYTIC SOLUTIONS OBTAINED IN THE ELECTROLYTIC PRODUCTION OF Filed Feb. 16, 1970 8 Regenerabzd A Elacimlytc MANGANESE DIOXIDE 5 Sheets-Sheet 5 E Lcc troLybc I l 100 Temperature [C] Temperaiw'e coefl'icieni 0f solubility afcalcz'um Sulfide di/zydrcde in Zea/mica! eleclrogyies usedfar cuwdz'c precipiiation of magcmese dioxide United States Patent US. Cl. 20483 8 Claims ABSTRACT OF THE DISCLOSURE Aqueous electrolytic solutions containing manganese sulfate, calcium sulfate and sulfuric acid, obtained in the electrolytic production of manganese (1V)-oxide at elevated temperatures in electrolytic cells are regenerated. The electrolytic solution is withdrawn from the electrolytic cell, cooled down to temperatures at least 5 centigrade degrees lower than the electrolysis temperature, precipitated matter is isolated therefrom after a period of at least 15 minutes, and the solution is recycled to the electrolytic cell.

The present invention relates to a process for regenerating aqueous electrolytic solutions containing manganese sulfate, calcium sulfate and sulfuric acid, obtained in the electrolytic production of manganese IV)-oxide at elevated temperatures.

Manganese dioxide is normally precipitated electrolytivally from a sulfuric acid manganese sulfate solution, at temperatures of between 80 and 98 C. To regenerate the electrolyte in the electrolytic cells, which is termed cell electrolyte hereinafter, it is necessary for an electrolyte portion to be withdrawn therefrom and to be neutralized by means of commercial manganese (III) -oxide prepared from crude manganese ore. The resulting solution which is termed regenerated electrolyte hereinafter, is recycled to the cell electrolyte.

Depending on its source of origin, manganese ore is often found to contain rather considerable proportions of calcium compounds. These may appear in commercial manganese (ID-oxide produced therefrom, in the form of carbonates, sulfates, oxides or even silicates, for example. Upon the use of cell electrolyte for ore-extraction, the above compounds are decomposed by the sulfuric acid contained in the electrolyte with the resultant formation of an equivalent proportion of calcium sulfate. As a result, regenerated electrolyte solutions saturated or supersaturated with calcium sulfate are obtained.

supersaturation may occur to a considerable extent up to a value 100 percent higher, for example, than the saturation concentration. If such saturated or supersaturated solution is fed to the cell electrolyte balance portion in the electrolytic cell, calcium sulfate commences to be precipitated during the electrolysis at various places in the electrolytic system. As time goes on, the calcium sulfate is found to reduce the cross-sectional area of pipe systems and to deposit on the cell walls and even on the cathodes, which is highly undesirable.

These undesirable phenomena can be obviated by increasing the crude ores natural calcium content by the addition of calcium compounds. As taught in US. Pat. 2,424,958, this can be done, for example, by first subjecting the cell electrolyte to incomplete neutralization using commercial manganese (ID-oxide. Following this, the neutralization is continued and completed by means of 3,030,862 Patented Dec. 28, 1971 a calcium compound, such as calcium carbonate, calcium oxide or calcium hydroxide.

A further method for neutralizing the supersaturation has been reported in US. Pat. 2,820,749. As described therein, the electrolyte for use in the electrolytic production of metallic manganese is prepared substantially in analogous manner, save that the extraction step is carried out at room temperature and the calcium sulfate-supersaturation is neutralized by inoculation, using calcium sulfate-dihydrate crystals at a pH-value of between 4.5 and 7.5. This process principle is also applicable in analogous manner to extraction processes which are carried out at temperatures outside the range set forth hereinabove. The addition of basic calcium compounds entails the loss of an equivalent amount of sulfuric acid for reaction with manganese (ID-oxide, whereas the formation of a solution always saturated with calcium sulfate is a disadvantage common to the two processes referred to hereinabove.

There is a wide variety of substances whose solubility is merely slightly temperature responsive or whose solubility even decreases consistently with an increasing temperature. These substances inter alia include calcium sulfate dihydrate CaSO -2H O, of which the solubility gradient in pure water increases slightly up to about 40 C. to then slightly decrease again. The two other crystalline forms of calcium sulfate, namely the semihydrate, CaSO /2H O, and the anhydrite, CaSO -OH O, both have negative temperature coeflicients. The semihydrate is the crystalline form with the lowest stability and fails to be formed under the conditions reported, whilst the anhydrite, which is the stablest crystalline form at temperatures above 41 C., has a very small nuclei-formation rate. In other words, it would be a commercially unattractive procedure to effect reduction of the calcium ion-content of the solutions by the transformation of CaSO -2H O precipitate to the anhydrite, in view of the long periods that would be needed to achieve this.

The cell electrolyte and regenerated electrolyte are, however, strongly concentrated electrolytic solutions containing manganese sulfate together with magnesium sul fate and potassium sulfate, depending on the source of origin of the starting material, for example up to 0.5 mol/ liter magnesium sulfate in those cases in which African crude manganese dioxide is the starting material. In addition thereto, the cell electrolyte contains sulfuric acid in a rather high concentration of between about 0.2 and 1.0 mol/liter. In view of this, the salts and sulfuric acid aforesaid were tested as to their influence upon the temperature responsive solubility of CaSO -2H O. The tests have unexpectedly shown that CaSO -2H O has a positive temperature coefficient of solubility in manganese sulfate solutions, which is the higher the higher the concentration of manganese sulfate therein (cf. FIG. 1 of the accompanying diagrams). Tests were also made on metal sulfates other than CaSO -2H O, but these could not be found to behave in analogous manner. In the presence of a 0.7 molar sodium sulfate solution, the temperature gradient for the solubility of CaSO '2H O was found to remain substantially unchanged, aside from a parallel displacement to slightly higher values. In a 0.25 molar magnesium sulfate solution, the calcium sulfate also could not be found to be substantially temperature responsive (cf. FIG. 2 of the accompanying diagrams).

The strong influence of sulfuric acid on the temperature responsiveness of the solubility of CaSO -2H O is illustrated in FIG. 3 of the accompanying diagrams. As shown therein, the presence of H 50 in a concentration of merely 0.1 mol produces at C. a solubility approximately percent higher than that at 20 C.

If the influence exerted by the sulfuric acid and manganese sulfate would additively add together, the cell electrolyte would always be found to have a solubility for CaSO -2H O much higher than that of the regenerated electrolyte, and it would then be impossible for material to be deposited in the cell electrolyte cycle. Comprehensive investigations have shown, however, that the solubility of CaSO -2H O in sulfuric acid solutions not only fails to be intensified but is even considerably reduced in the presence of neutral sulfates (cf. FIG. 4 of the accompanying diagrams). As a result of these influences, which partly add together and partly compensate each other, the temperature responsiveness of the solubility of caso,-2H o in a commercial cell electrolyte is practically parallel, despite the relatively high concentration of acid therein, with that in the regenerated electrolyte, with some more or less great displacement, depending on the electrolytes exact composition.

The process of the present invention for regenerating an aqueous electrolytic solution containing manganese sulfate, calcium sulfate and sulfuric acid, obtained in the electrolytic production of manganese (IV)-oxide at elevated temperatures in an electrolytic cell, comprises more particularly withdrawing the electrolyte solution from the electrolytic cell, cooling the solution down to temperatures at least 5 centigrade degrees, preferably 14 to 40 centigrade degrees lower than the electrolysis temperature, isolating precipitated matter therefrom after a period of at least 15 minutes, preferably 1 to 3 hours, and recycling the said solution to the electrolytic cell.

A preferred feature of the present invention comprises subjecting the electrolytic solutions, prior to cooling them, to treatment with manganese (II)-oxide or manganese (ID-carbonate to effect partial or complete neutralization of the sulfuric acid therein and, in the case of partial neutralization, neutralizing residual sulfuric acid by means of calcium oxide, calcium hydroxide or calcium carbonate.

A further preferred feature of the present invention comprises neutralizing the electrolytic solution, freeing it from undissolved solid matter and then cooling it with agitation.

A still further preferred feature of the present invention provides for the quantities of water evaporated during the electrolysis and regeneration of the electrolyte to be replaced with corresponding quantities of fresh water, the water being added to the electrolytic solution following isolation of precipitated matter and prior to recycling the said solution to the electrolytic cell.

Room temperature is the lower temperature limit down to which the electrolytic solution should conveniently be cooled. Needless to say it is also possible for the solution to be cooled down to temperatures lower than room temperature, but this would imply the use of efficient cooling means and render the process a commercially less attractive procedure.

The solubility curves shown in FIG. 5 of the accompanying diagrams illustrate the basic idea underlying the process of the present invention. The neutralization of the cell electrolyte portion withdrawn from the electrolytic cell by means of crude manganese (ID-oxide effects in the so-called regenerated electrolyte the formation of a suspension comprising metal hydroxides, silicates, sulfates, further compounds, and CASO -2H O which may be found to have been precipitated. As the saturation solubility of the cell electrolyte is a function of its temperature (point A), the suspension is found to at least cool down to a temperature at which the same saturation concentration is reached (point B). After having been allowed to stand over a suitable period of time, e.g. 3 hours, the regenerated electrolyte is filtered to free it from undissolved or precipitated matter and recycled then to the cell electrolyte cycle.

In this manner, it is possible to establish in the regenerated electrolyte a concentration of calcium ions corresponding to between about 50 and 100% of the solubility of CaSO -2H O in the cell electrolyte, at 98 C. For rea- 4 sons of a satisfactory heat balance, it is generally sufiicient to produce minor undersaturation in order to avoid undesirable precipitation of further dissolved salts.

FIG. 5 shows the solubility curves for CaSO -2H O for the electrolyte described in Example 1 hereinafter. As shown therein, the curve of the cell electrolyte is situated below the curve for the regenerated electrolyte. Thus, it is possible by inoculation with CaSO -2H O crystals at a given working temperature to always produce a regenerated electrolyte supersaturated with respect to the cell electrolyte. In other words, a cell electrolyte supersaturated with calcium sulfate would be obtained if the two electrolytes were mixed together.

EXAMPLE 1 The starting material was a cell electrolyte. It contained 110 grams MnSO 70 grams H grams MgSO and 3.5 kg. K 50 per liter and had a temperature of 95 C. The electrolyte was mixed in an agitator vessel with reduced manganese dioxide until the free sulfuric acid was found to have been substantially neutralized. It was immaterial whether the last residues of the free acid were neutralized with reduced crude manganese dioxide or with a further basic material, for example calcium carbonate, calcium hydroxide or sodium carbonate or a similar compound. The neutralized electrolyte was cooled with agitation, within 3 hours, down to a temperature of C., and filtered. The resulting regenerated electrolyte was recycled to the cell electrolyte cycle. CaSO -2H O could not be found to be precipitated therein.

EXAMPLE 2 The starting material was a cell electrolyte. It contained 100 grams MnSO 50 grams H 50 and 20 grams MgSO per liter and had a temperature of C. The cell electrolyte was treated in the manner set forth in Example 1 using reduced crude manganese dioxide or, e.g. natural rhodochrosite (MnCO and the resulting suspension was conveyed from the agitator vessel to a settling tank, in which it was cooled down to about 40 C. and allowed to stand for some hours. Following filtration, the regenerated electrolyte was ready for use.

What is claimed is:

1. A process for regenerating an aqueous electrolytic solution containing manganese sulfate, calcium sulfate and sulfuric acid, obtained in the electrolytic production of manganese (IV)-oxide at elevated temperatures in an electrolytic cell, which comprises withdrawing the electrolytic solution from the electrolytic cell, cooling the said solution down to temperatures at least 5 centigrade degrees lower than the electrolysis temperature, isolating precipitated matter therefrom after a period of at least 15 minutes, and recycling the said solution to the electrolytic cell.

2. The process as claimed in claim 1, which comprises cooling the said electrolytic solution down to a temperature 15 to 40 centigrade degrees lower than the electrolysis temperature.

3. The process as claimed in claim 1, which comprises isolating precipitated matter from the cooled electrolytic solution after a period of between 1 and 3 hours.

4. The process as claimed in claim 1, which comprises subjecting the electrolytic solution withdrawn from the electrolytic cell to treatment with manganese (ID-oxide or manganese (ID-carbonate to effect partial or complete neutralization of the sulfuric acid therein, prior to cooling the said solution.

5. The process is claimed in claim 4, wherein the sulfuric acid is partially neutralized and residual sulfuric acid is neutralized by means of calcium oxide, calcium hydroxide or calcium carbonate, prior to cooling the electrolytic solution.

6. The process as claimed in claim 1, which comprises neutralizing the electrolytic solution, freeing it from undissolved matter and then cooling it.

7. The process as claimed in claim 1, wherein the electrolytic solution is cooled with agitation.

8. The process as claimed in claim 1, wherein the quantities of water evaporated during the electrolysis and regeneration of the electrolyte is replaced by corresponding quantities of fresh water, the water being added to the electrolytic solution following isolation of precipitated matter and prior to recycling the said solution to the electrolytic cell.

References Cited UNITED STATES PATENTS 2,424,958 8/ 1947 Clemens 20483 2,820,749 1/ 1958 Carosella 23l22 X 3,065,155 11/1962 Welsh 204-83 3,436,323 4/1969 Shimizu et a1. 204-96 JOHN H. MACK, Primary Examiner 10 D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 23-1l7; 204-96 

